The supply and distribution of electrical power to various loads relies upon numerous different arrangements to accomplish that end. One area that is receiving increased attention is the provision of electrical power to various, typically stationary, loads not only from an AC power source such as the conventional AC electric utility grid (simply, “the AC grid”) or a generator, but additionally from power sources such as fuel cells, microturbines, solar cells, and the like. Often the conventional AC electric grid serves as one source, but it is “partnered” with another source such as a fuel cell, etc. Indeed, some such systems rely on the fuel cell or microturbine or the like to provide the base load requirements for a dedicated group of customer loads, as for instance a hospital, mall, etc., and use the AC grid for peak requirements or as a back-up alternative.
Of course, such arrangements require the appropriate and sometimes complex and/or costly distribution and switching arrangements for dealing with such diverse power sources. This is further complicated by the fact many such power sources provide their power output as a direct current (DC), whereas the conventional grid provides, and many, most, or all of the customer loads are viewed as relying upon, alternating current (AC). This is traditionally the situation with a fuel cell power source, for example, which provides a DC output that is then converted, as by an inverter, to AC for connection to the customer's loads, often in parallel with the AC power grid. Examples of such power distribution arrangements may be found in U.S. Pat. Nos. 6,465,910; 6,757,590; and 7,061,139, for example, all assigned to the assignee of the present application.
Typical in most such arrangements, as depicted in the block diagram of FIG. 1, is the provision of a power system 10 having an AC power source, such as the AC grid 12 as one power source, and one or more further power sources 14 typically providing, at least initially, DC power. In the example illustrated in FIG. 1 and for the purposes of further discussion of the disclosure, that source of DC power will appear and be described in the context of a fuel cell 14. However, it will be appreciated that power systems employing other types of power sources such as microturbines, photovoltaics, and the like may, to the extent they supply DC power that undergoes conversion to AC for connection to the customer load, similarly benefit from the disclosure.
Referring further to FIG. 1, the AC grid 12 and the fuel cell 14, either independently (in alternation) or in parallel combination, supply power to the so-called “customer's load” 16, typically in AC form. The fuel cell 14 serves as a source of DC power which is connected from its output, as via leads 20A and 20B, to the DC input of a converter 22, here depicted as inverter 22, for conversion from DC to AC. Although the AC power from the AC grid 12 and from the inverter 22 might typically be three-phase, for the simplicity of depiction in FIG. 1 and later with respect to the invention, it is shown as appearing on single electrical leads, with the other leads being assumed. Similarly, with respect to various of the control circuitry aspects of FIG. 1 and the disclosure, it will be understood that single-line representations have sometimes been used for twisted pairs, or grouped parallel leads, which serve as signal conductors.
The AC output from the inverter 22 appears on lead 24, which includes branches 24A and 24B. Branch 24A is depicted as including a grid-independent (G/I) switch 26 and extends to node 27. Similarly, the AC from the grid 12 appears on lead 28, which includes branches 28A and 28B. Branch 28A is depicted as including a grid-connect (G/C) switch 30 and extends to the node 27. A common lead 32 extends from node 27 to the customer loads 16 to supply them with AC power from either or both of the power sources 12 and 14, as determined by the states of the G/I and G/C switches 26 and 30 respectively. Typically, in steady state operation, at least the G/I switch 26 is normally closed to supply power from the fuel cell 14 and the G/C switch 30 may also be closed to simultaneously supply power in parallel from the AC grid 12. Any excess power supplied by the fuel cell 14 may be directed to the grid 12. If the G/C switch 30 is open, the fuel cell operates in the grid independent mode as the sole power supply to the customer's load(s) 16.
Importantly, there are associated with the operation of the fuel cell 14 power system a number of additional electrical loads, here generally designated as “auxiliary loads” 34. The auxiliary loads 34 may include fans, pumps, blowers, heaters, etc, as well as the controllers for the electronic control system (ECS) and the power conditioning system (PCS), here cumulatively shown as controller 34A, and may be AC and/or DC-type loads. Indeed, the auxiliary loads typically are, or have been, a mix of both AC and DC loads. An automatic transfer switch (ATS) 40, here illustrated as a single pole, double throw switch, has its common terminal connected via lead 42 to the auxiliary loads 34 to provide the power input thereto. The transfer switch 40 then serves to connect the auxiliary loads 34 either to the AC output of inverter 22 via branch lead 24B or to the AC output of the AC grid 12 via branch lead 28B. It will be noted that in either instance, the power supplied to the input of the function block designated “auxiliary loads 34” is AC, yet some of those loads are of the DC type and require further conversion of the AC power to DC power. This conversion/rectification may be provided internally of the function block 34 representing the auxiliary loads and, though not separately shown, will be understood to require the appropriate additional controls and/or circuitry.
The automatic transfer switch 40 includes control circuitry structured and connected to sense the presence of voltage on either branch lead 24B or branch lead 28B, with a bias toward branch lead 24B, and to close that circuit if voltage is present. During initial start-up of the fuel cell 14 power system, however, both the G/I switch 26 and the G/C switch 30 are in the “open” condition and there is no voltage on the branch lead 24B from the AC output of inverter 22. Thus, the ATS 40 closes the circuit through the branch lead 28B with the AC grid 12 to provide initial power to the auxiliary loads 34 until the fuel cell 14 power system is operating, whereupon the ATS 40 switches to the branch lead 24B to power the auxiliary loads. Additionally, appropriate controllers and operating algorithms, not shown, then regulate (close and/or open) the respective G/I switch 26 and G/C switch 30 to connect AC power to the customer loads 16 from one, the other, or both power sources 12 and 14.
While providing satisfactory operation, the afore-described arrangement includes several limitations. The ATS 40 is subject to occasional failure due to the mechanical switching between lines 24B and 28B. The same may be said of the G/I switch 26, though perhaps to a lesser extent. Also, the sequence of converting the DC from the fuel cell 14 to AC via the inverter 22 and then re-converting (rectifying) a portion of that back to DC to power those of the auxiliary loads 34 that require DC is an additional burden on system efficiency. This is particularly noteworthy when considered in the context that it is generally desirable to maximize the percentage of DC auxiliary loads relative to AC auxiliary loads for flexibility in adapting systems for use in differing countries because the voltages and frequencies for AC systems vary. Stated another way, the fewer the auxiliary loads that are AC powered, the fewer the number of such AC loads that need to be changed or customized for use in different countries, and thus a reduction in the costs that attend such needs for changes.